It is customary in the pulp and paper industry when producing chemical pulp to burn the used pulping liquor, i.e. the black liquor in a chemical and heat recovery furnace. In a conventional recovery furnace the process chemical is recovered by injecting black liquor at a dry solids content of about 65% into the furnace. As the liquor enters the furnace, it rapidly dries and burns under reducing conditions above the hearth whereby a smelt is formed which is discharged from the furnace. Oxygen is added in form of secondary air to burn the gases generated at a higher location in the furnace. The furnace is provided with heat exchange surfaces to recover heat and generate steam.
It is desirable that as much water as possible is removed from the black liquor before it is introduced into the recovery furnace as evaporation of the residual water of the black liquor in the furnace causes cooling and slowing of the combustion process. This involves, however, difficult handling problems. Black liquor goes into a semi-solid state at a dry solids content of about 60% and at about 80% DS it has a viscosity exceeding 500 cP at 100.degree. C.
It is the prime object of the present invention to provide a method for concentrating black liquor to a high dry solids content, preferably to about 100%.
Earlier attempts to reduce the water content of the black liquor which is introduced into the recovery furnace have been made. U.S. Pat. No. 4,347,220 discloses a method in which sawdust is mixed with partially concentrated black liquor and dried by heated air.
It has also been proposed to evaporate water from black liquor by mixing the black liquor which has been previously concentrated by conventional evaporating methods with superheated steam under pressure. The released vapor is removed, heated and recycled. When the water content of the black liquor has been reduced by this treatment 0-20%, the black liquor is exhausted from the apparatus through a nozzle, whereby, as the pressure decreases, the remaining water vaporizes and a particulate, dry and non-sticky substance which can be burned in a furnace is produced. Such a method is described in the Swedish patent specification 119215.